Several studies over the past two decades have investigated the neuropsychological deficits in children with attention deficit hyperactivity disorder (ADHD), but much less has been done on adults. This study aimed to assess the deficits in executive functions of adults with ADHD, especially in the areas of attention, inhibition, impulsivity, and planning. Twenty-four adults (18 years and older) diagnosed with ADHD according to the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition criteria, and also assessed with Conners' Adult ADHD Rating Scale, participated in the study. Executive functions in cases were compared with 20 matched controls through the three instruments of Tower of London (TOL), Continuous Performance Test (CPT), and Stroop test. Performance of cases was weaker than that of the control group in TOL. The difference was significant in subsequent thinking time in most of the trials and number of movements only at level 2. In CPT, the adults with ADHD made more commission errors. In addition, the ADHD cases made more errors in the word card of the Stroop test, and the time they spent reading all three cards was significantly longer than that of the control group. Our study suggests that several deficits in executive functions related to ADHD persist into adulthood, such as impairments in planning time and set shifting, response inhibition, impulsivity, and visuolingual processing. However, simple (visual-motor processing) and sustained attention might improve with age.